1. Field of the Invention
The present invention relates to a fuel supply system for an automotive engine.
2. Description of the Related Art
It is known in the art that the temperature of an exhaust gas from the engine rises in response to an increase in the engine speed and engine load, and in response to a retardation of the spark advance, so that a continued high-speed heavy load operation of the engine will result in a temperature rise of an exhaust system. It is also known that, assuming the engine speed, engine load and spark advance to be constant, the exhaust gas temperature reaches a maximum value when the engine is operating with a combustible mixture having a stoichiometric air/fuel ratio. The exhaust gas temperature becomes lower as the air/fuel ratio becomes less than the stoichiometric ratio, i.e., as the combustible mixture is enriched.
Overheating of the exhaust system must be avoided because it adversely affects the exhaust gas purifier and turbocharger provided in the exhaust system. Therefore, it has been customary to enrich the combustible mixture during the high-speed heavy-load condition of the engine in order to lower the exhaust gas temperature below an acceptable point, while normally operating the engine with a stoichiometric combustible mixture during the steady speed condition.
From the viewpoint of fuel economy, however, it is desirable to minimize the needs for an enriched combustible mixture. To this end, based on the recognition that there is a certain time delay before the exhaust gas temperature is raised during the transitional condition of the engine, it has been proposed in the prior art that enrichment of combustible mixture be postponed for a predetermined delay time. For example, Japanese Unexamined Patent Publication No. 58-51241 discloses a fuel supply system wherein a stoichiometric mixture is supplied during a steady speed condition and the mixture is enriched only upon elapse of a time delay after the engine load is increased during a transient condition. In this system, the delay time is varied in accordance with the engine load and engine speed, in such a manner that, under a heavy load condition wherein the temperature rise in the exhaust system occurs in a shorter period, the delay time is correspondingly shortened to avoid undesirable overheating. Also, Japanese patent application No. 59-174017 filed Aug. 23, 1984 proposes to vary the fuel enrichment delay time in response to coolant temperature.
In those fuel systems wherein the enrichment of the combustible mixture is delayed, the temperature rise in the exhaust system will be prohibitive if the delay time is set for a larger value and, conversely, the fuel consumption will be adversely affected if the delay time is made shorter. That is, the delay time must meet two opposing requirements for reducing the temperature of the exhaust system and for fuel economy, and it has been difficult to satisfy both requirements. This problem will be discussed in more detail with reference to FIG. 7 wherein a composite time chart is shown illustrating the function of a typical fuel supply system adapted to enrich the combustible mixture for the purpose of suppressing the exhaust gas temperature rise during the transient condition. In the chart of FIG. 7, curve (a) represents the running mode of a vehicle in terms of the vehicle speed indicated by the ordinate. Curve (a) indicates that the vehicle has undergone four acceleration cycles during the illustrated running mode. Curve (b) represents the rate of fuel enrichment with respect to the stoichiometric mixture, which rate is computed in accordance with the engine load, engine speed and other engine parameters. Curve (c) indicates the count of a delay counter for counting the fuel enrichment delay time. Two alternative values A and B are shown as indicating the preset value for the counter, meaning that fuel enrichment is performed as shown in curve (d) when the counter counts over the preset value A or B. It will be understood that if the smaller preset value B is selected, the combustible mixture is enriched for each acceleration cycle as shown by the dotted line in curve (d), resulting in an increased fuel consumption. Conversely, if the larger value A is selected as the preset value for counting over the delay time, then fuel enrichment is not performed in the first, second, and fourth acceleration cycles, and the mixture is enriched as shown by the solid line of curve (d) only in the third acceleration cycle wherein acceleration is continued for an extended period, thereby resulting in the imposition of a drastic heat load on the exhaust system. It will be noted that, when the vehicle running mode is such that the first and second acceleration cycles as shown by curve (b) are repeated, no fuel enrichment is performed at all, thereby causing a danger that the exhaust system will overheat.